Police in Austin, Texas, reported that the city’s fourth bomb in less than a month shows similarities to the previous blasts but indicates a “significant change” in the “serial bomber’s” tactics and level of sophistication. Sunday night’s device appears to have been detonated by a tripwire when two young men walked nearby.

Austin police and EMS units responded to what they called a “Bomb Hotshot” call Sunday night. EMS officials tweeted confirmation of transporting two people with serious but “non-life-threatening” injuries.

Police in Texas increased the reward being offered for information leading to the arrest of the Austin package bomber. Since March 2, the bomber or bombers killed two people and injured two others in three separate attacks.

Media reports indicate that investigators in Austin, Texas, are developing theories of what might have motivated a bomber or bombers to set off three explosions in residential communities this month. Some neighbors are offering possible motives for the attacks. The three bomb blasts left two people dead and two others seriously injured.

Authorities in Austin, Texas, identified Draylen Mason as the 17-year-old male who died in the second package bombing on Monday. Mason is being remembered as a musician and friend. The blast also wounded his mother who remains hospitalized in stable condition.